Astronomical Images : Orbs, axes and poles of the superior planets

Gregor Reisch

Astronomical Images

<p style='text-align: justify;'>The <i>Margarita philosophica</i> was a compendium, or 'Epitome' of university learning in the sixteenth century. It was written by the prior of the house of Carthusians at Freiburg, Gregor Reisch (d. 1525), and was first published in 1503 in Freiburg by Johannes Schott, a printer from Strasbourg. The work was illustrated amply with somewhat crude woodcuts, and was divided into twelve books, with one book each dealing with the trivium (grammar, dialectic and rhetoric) and quadrivium (arithmetic, music, geometry, astronomy), four books devoted to natural philosophy, and one book on moral philosophy. It was a popular work, reprinted numerous times during the sixteenth century, including the unauthorized, augmented editions by another printer at Strasbourg, Johann Grueninger. Oronce Fine edited and added to the Latin text of the 1535 edition. In the 1512 edition, Grueninger attached an appendix consisting of material not discussed extensively in the original Schott edition. This appendix included Greek and Hebrew alphabets, musical notation, perspective and architecture, and explanation of such instruments as the quadrant, astrolabe, and torquetum. In book 7, the <i>Margarita</i> includes a summary of Peuerbach's <i>Theoricae novae planetarum</i> and reproduces some of its original diagrams. This woodcut is a synthesis of the diagram of the orbs of the superior planets (sig. Q8v) and of the diagram of the axes and poles of the movements of the superior planets. The outermost circle represents the zodiac, and the three orbs of Saturn are represented as in the diagram of the orbs of the superior planets (sig. Q8v), except that the equant circle and the deferent circle on which the centre of the epicycle is attached are not drawn. The epicycle is represented (<i>epicyclus</i>), and the planet in it (<i>planeta</i>), in two different positions shown by two radii. On the left side, we must probably read <i>pars superior</i>, meaning that in this position the planet is in the superior part of the epicycle. The text specifies that the body of the planet has a movement in longitude eastward (<i>secundum successionem</i>) when it is in the superior part of the epicycle, whereas this movement is westward in the inferior part. Consequently, 'the planet is said to be direct in the superior part, but retrograde in the inferior part' (<i>planeta in parte superiori directus, in parte vero inferiori retrogrades dicitur</i>). The line of the apogee (<i>linea augis</i>), which is also the line indicating the plane of the deferent (<i>superficies plana deferentis</i>), crosses at right angles at the centre of the deferent (<i>[c.] deferentis</i>), the axis of the deferent, whose poles are indicated (<i>polus deferentis</i>). This line of the apogee successively passes by the apogee (<i>aux</i>), the centre of the equant (<i>c. equantis</i>), the centre of the deferent (<i>[c.] deferentis</i>), the centre of the World (<i>c. mundi</i>) and the perigee (<i>oppositum augis</i>). The same line intersects at the centre of the World (<i>c. mundi</i>) a vertical line indicating the plane of the ecliptic (<i>superficies plana eclipticae</i>), which crosses at right angles at the centre of the World the axis of the ecliptic. The poles of this axis are marked (<i>polus eclipticae</i>): the one northward (<i>Septentrio vel Aquilo</i>), the other southward (<i>Meridies vel Auster</i>). The intersection between the axis of the ecliptic and the axis of the eccentric deferent is at a notable distance from the centre of the World. As a consequence, in Peuerbach's words, when the eccentric deferent 'moves eastward on its axis, ... its poles are separated from the poles of the zodiac by unequal distances' (<i>poli eius distant a polis zodiaci distantia non aequali</i>). The diagram actually shows that the south pole of the deferent is less distant than the north pole from the axis of the ecliptic. The only problem is that it ought to be the reverse: according to Peuerbach, the north pole is less distant than the south pole. The <i>Margarita</i> diagram accurately reproduces the diagram in the first edition of the <i>Theoricae novae</i>, but inverts the indications of the north and south. Peuerbach adds that for the same reason, 'the apogees of the deferent orbs of the epicycles' (<i>auges deferentium epicyclos</i>), like the 'perigees, centres and poles of [these] eccentric deferents' (<i>opposita atque centra, et poli deferentium eccentricorum</i>) describe 'circumferences parallel to the plane of the ecliptic' (<i>circumferentias superficiei eclipticaeâ?¦ aequidistantes</i>), as the orbs are carried by the motion of the eighth sphere. These rotations are represented in the diagram by three lines parallel to the plane of the ecliptic (<i>superficies plana eclipticae</i>) drawn from the two poles of the eccentric deferent and the centre of the eccentric deferent to points symmetrical in relation to the axis of the ecliptic, which is the axis of rotation. In the diagram of Peuerbach's first edition, the line from the centre of the eccentric deferent is not drawn but there are two additional lines drawn from the points of the apogee (<i>aux</i>) and perigee (<i>oppositum augis</i>) of the eccentric deferent. For clearer diagrams of the axes and poles of the superior planets, see the commentaries by Reinhold (1553, fol. 42r) and Schreckenfuchs (1556, plate after p. 98). The spatial relations of the axes of the superior planets are more clearly shown in the three-dimensional diagram provided in Erasmus Oswald Schreckenfuchs' Commentaria in novas theoricas planetarum Georgii Purbachii (Basel: Henricus Petri, 1556), plate after p. 98. Translated quotations of Peuerbach's <i>Theoricae</i> are from Aiton (1987). Quotations from <i>Margarita philosophica</i> are translated by Isabelle Pantin.</p>